1. Field of the Invention
The present invention relates to preloaded pneumatic air bearings.
2. Background
A bearing is a device that reduces friction between moving parts, or supports moving loads. There are two main types of bearings. The anti-friction bearing minimizes friction using devices such as roller bearings or ball bearings. The friction or sliding bearing, on the other hand, minimizes friction using active lubrication or other means to facilitate motion between moving parts. Many bearing assemblies take advantage of both principles—e.g., a lubricated ball bearing assembly.
A pneumatic bearing is an example of a friction or sliding bearing. It uses compressed gas to create a consistent gas film upon which the bearing rests and moves. The gas film acts as a virtually frictionless lubricant that facilitates smooth motion between the pneumatic bearing and the surface upon which it rests. The bearing surface upon which the lubricating gas film is generated is called the “active surface.” Typically, pneumatic bearings require at least a steady source of compressed gas to maintain the lubricating gas film. Additionally, pneumatic bearings are often “pre-loaded” to provide stiffness. A pre-load force opposes the lift force generated by the lubricating gas film. Stiffness is a measure of how much additional force, applied in a direction normal to the active surface, would be required to change the thickness of the lubricating gas film by a certain amount. A pre-load may be provided by gravity with, for example, weights. A pre-load may also be provided with magnetic force, or may be generated by oppositely positioned active surfaces. A significant preload force is sometimes required depending upon the required degree of stabilization and stiffness.
An exemplary environment for pneumatic bearings is in the semi-conductor lithography field. There, pneumatic bearings provide a number of advantages. Pneumatic bearings are virtually frictionless, and therefore produce no particulate wear materials as they operate. Such particulate matter would be troublesome in the ultra-clean semiconductor manufacturing environment. Additionally, lubricants present in ball or roller bearings could outgas contaminant molecules, which are also detrimental in semiconductor manufacturing environments. Pneumatic bearings also require relatively little maintenance or regular repair. Finally, properly preloaded pneumatic bearings provide sufficient stiffness for the precise tolerances required in the scanning stages of semiconductor lithography tools.
Stiffness measured in the direction parallel to the active surface—i.e., “in-plane” stiffness—is ideally zero. The lower the actual in-plane stiffness, the better the air bearing is able to isolate its payload from in-plane base vibrations. Because the air film itself cannot support a shear load, it intrinsically has zero in-plane stiffness. All in-plane stiffness is therefore “parasitic” stiffness caused by external connections to the bearing such as the bending stiffness of gas hoses feeding the bearing. Parasitic stiffness is detrimental to the bearing's ability to isolate its payload from in-plane base vibrations. One way to mitigate parasitic stiffness is to reduce the number of external connections to the pneumatic bearing.
Stiffness measured in the direction perpendicular to the active surface—i.e., “out-of-plane” stiffness—is ideally very high. Out-of-plane vibrations are transmitted to the payload directly through the air film stiffness. Therefore, air bearings cannot completely isolate the payload from out-of-plane vibrations. One way to mitigate out-of-plane vibration is to make the air film sufficiently stiff so that the resonant frequency of the pneumatic bearing payload system is much higher than the excitation frequency of the out-of-plane vibrations. If this characteristic is achieved, the system response does not become amplified by resonance. In the context of semiconductor lithography tools, a typical out-of-plane stiffness may be on the order of millions of pounds per inch.
As noted above, one drawback to pneumatic bearings used in semiconductor lithography tool context are the number of physical connections required for practical operation. A pneumatic bearing requires at least a source of compressed air. Additionally, in the semiconductor lithography tool field, pneumatic bearings often support and carry wafers, reticles, and other payloads whose dynamic positioning in all four dimensions (x, y, z, time) must be precisely controlled. Such control requires precise positioning means for the pneumatic bearing. Thus, the pneumatic bearing will often be required to support numerous physical connections, either directly, or by virtue of physical connections to the pneumatic bearing payload.
For example, if pre-loading is accomplished with electromagnets, wires will be required to supply electricity. If pre-loading is accomplished using a counteracting vacuum on the active surface, then an additional connection would be necessary to connect to a vacuum source. Additionally, the bearings may also contain positioning aids such as interferometers, which may also require a physical connection to the pneumatic bearing. As the number of physical connections to the pneumatic bearing grows, so does unwanted drag, vibration and parasitic stiffness on the bearing.
Improvements in pneumatic bearing design are constantly needed. This is especially true in the semiconductor lithography tool arts, where manufacturing tools are being constantly pushed to more precise tolerances and faster speeds.
The present invention will now be described with reference to the accompanying drawings.